Business Segments (Segment Financial Information) (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
External customer revenues		$ 1,427.3	$ 1,612.8	$ 1,716.5
Intersegment revenues		0.0	0.0	0.0
Total revenues		1,427.3	1,612.8	1,716.5
Fuel Costs		268.8	259.8	304.5
Amortization of intangibles		0.0	0.0	1.2
Depreciation and amortization		132.3	134.6	135.6
Goodwill impairment (Note 7)		0.0	317.0	0.0
Interest expense		106.1	118.3	126.6
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest		(793.3)	(231.4)	72.6
Net loss from continuing operations		(514.5)	(251.4)	57.2
Discontinued operations, net of tax		29.3	12.4	(131.8)
Net income (loss)		(485.2)	(239.0)	(74.6)
Cash capital expenditures		148.5	137.2	118.1
Total assets (end of year) (a)	$ 2,419.2	2,419.2	3,324.7	3,559.1
Fixed-asset impairment (Note 15)	623.5	859.0	0.0	11.5
Operating Segments [Member] | Transmission and Distribution [Member]
Segment Reporting Information [Line Items]
OVEC Revenue			19.7	32.5
External customer revenues		806.7	855.5	1,020.1
Intersegment revenues		1.3	1.5	1.7
Total revenues		808.0	857.0	1,021.8
Depreciation and amortization		71.0	71.5	75.5
Goodwill impairment (Note 7)			0.0
Interest expense		24.7	28.9	29.8
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest		143.0	188.1	241.7
Cash capital expenditures		83.4	98.3	100.4
Total assets (end of year) (a)	1,710.5	1,710.5	1,688.8	1,686.1
Fixed-asset impairment (Note 15)		0.0		0.0
Operating Segments [Member] | Generation [Member]
Segment Reporting Information [Line Items]
External customer revenues		611.5	770.3	721.8
Intersegment revenues		0.0	186.6	72.8
Total revenues		611.5	956.9	794.6
Depreciation and amortization		55.4	72.6	75.3
Goodwill impairment (Note 7)			0.0
Interest expense		0.4	2.9	5.0
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest		(1,353.9)	(28.7)	(78.0)
Cash capital expenditures		64.2	35.2	14.5
Total assets (end of year) (a)	472.3	472.3	1,805.0	1,771.4
Fixed-asset impairment (Note 15)		1,353.5		0.0
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
External customer revenues		9.1	6.7	7.1
Intersegment revenues		5.7	4.2	3.8
Total revenues		14.8	10.9	10.9
Depreciation and amortization		5.9	(9.5)	(15.2)
Goodwill impairment (Note 7)			317.0
Interest expense		81.3	86.8	92.5
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest		417.6	(390.8)	(91.1)
Cash capital expenditures		0.9	3.7	3.2
Total assets (end of year) (a)	673.6	673.6	1,170.3	1,397.5
Fixed-asset impairment (Note 15)		(494.5)		11.5
Consolidation, Eliminations [Member]
Segment Reporting Information [Line Items]
External customer revenues		0.0	(19.7)	(32.5)
Intersegment revenues		(7.0)	(192.3)	(78.3)
Total revenues		(7.0)	(212.0)	(110.8)
Depreciation and amortization		0.0	0.0	0.0
Goodwill impairment (Note 7)			0.0
Interest expense		(0.3)	(0.3)	(0.7)
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest		0.0	0.0	0.0
Cash capital expenditures		0.0	0.0	0.0
Total assets (end of year) (a)	(437.2)	(437.2)	(1,339.4)	(1,295.9)
Fixed-asset impairment (Note 15)		0.0		0.0
THE DAYTON POWER AND LIGHT COMPANY [Member]
Segment Reporting Information [Line Items]
External customer revenues		1,365.9	1,552.3	1,668.3
Intersegment revenues		0.0	0.0	0.0
Total revenues		1,365.9	1,552.3	1,668.3
Fuel Costs		248.9	244.7	314.9
Depreciation and amortization		120.3	138.2	144.8
Interest expense		24.5	30.9	33.9
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest		(1,195.1)	141.5	154.7
Net income (loss)		(772.7)	106.4	115.0
Cash capital expenditures		128.3	127.0	114.2
Total assets (end of year) (a)	2,035.1	2,035.1	3,359.6	3,328.8
Fixed-asset impairment (Note 15)	496.4	1,353.5	0.0	0.0
THE DAYTON POWER AND LIGHT COMPANY [Member] | Operating Segments [Member] | Transmission and Distribution [Member]
Segment Reporting Information [Line Items]
OVEC Revenue			19.7	32.5
External customer revenues		808.0	857.0	1,021.8
Intersegment revenues		0.0	0.0	0.0
Total revenues		808.0	857.0	1,021.8
Depreciation and amortization		71.0	71.5	75.5
Interest expense		24.0	28.0	28.9
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest		143.6	189.0	242.6
Cash capital expenditures		83.4	98.3	100.4
Total assets (end of year) (a)	1,710.5	1,710.5	1,688.8	1,686.1
Fixed-asset impairment (Note 15)		0.0
THE DAYTON POWER AND LIGHT COMPANY [Member] | Operating Segments [Member] | Generation [Member]
Segment Reporting Information [Line Items]
External customer revenues		557.9	715.0	679.0
Intersegment revenues		0.0	186.6	72.8
Total revenues		557.9	901.6	751.8
Depreciation and amortization		49.3	66.7	69.3
Interest expense		0.5	2.9	5.0
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest		(1,338.7)	(47.5)	(87.9)
Cash capital expenditures		44.9	28.7	13.8
Total assets (end of year) (a)	324.6	324.6	1,670.8	1,642.7
Fixed-asset impairment (Note 15)		1,353.5
THE DAYTON POWER AND LIGHT COMPANY [Member] | Consolidation, Eliminations [Member]
Segment Reporting Information [Line Items]
External customer revenues		0.0	(19.7)	(32.5)
Intersegment revenues		0.0	(186.6)	(72.8)
Total revenues		0.0	(206.3)	(105.3)
Depreciation and amortization		0.0	0.0	0.0
Interest expense		0.0	0.0	0.0
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest		0.0	0.0	0.0
Cash capital expenditures		0.0	0.0	0.0
Total assets (end of year) (a)	$ 0.0	0.0	$ 0.0	$ 0.0
Fixed-asset impairment (Note 15)		$ 0.0